The invention concerns a process for the removal of impurities, in particular for the removal of sodium, from aluminum melts by means of reactive chlorine in a filter bed.
It is known that by treating aluminum melts with reactive chlorine the sodium content of the melt can be lowered to a level of a few ppm. The chlorine is mainly supplied to the melt in the form of a gas mixture containing an inert carrier gas, which has the function of transporting the resultant sodium chloride to the surface of the melt and of lowering the hydrogen content of the melt by lowering the partial pressure of hydrogen. The use of gaseous chlorine is difficult in practice due to the fact that this gas is harmful to the health. For this reason, therefore efforts have been made for a long time to diminish the quantity of chlorine to be converted and, at the same time, to raise the efficiency of the chlorine by using a surface active substance in place of the gas. Thus for example German Pat. No. 815 106 describes a process by which materials, preferably active carbon or silica gel, which have active, chlorine-charged surfaces, are added to the melt by means of a special container, or if desired by means of a normal immersion bell.
U.S. Pat. No. 737,303 suggests a process in which the liquid aluminum is passed through a filter bed of refractory material, through which reactive chlorine is made to pass either periodically or continuously in the direction counter to the flow of the aluminum.
In German Pat. No. 1 912 877 a process for the treatment of aluminum melts is proposed, whereby the molten metal is covered with a flux which is in the liquid state at the temperature of the treatment and is able to bind to it the major part of the NaCl resulting from the chlorine treatment. The fluxes which are used for this purpose are salt mixtures which contain alkali or alkaline earth halides and additions of a complex salt such as sodium hexafluor aluminate or the like.
The processes described in German Pat. No. 1 912 877 and U.S. Pat. No. 3,737,303 viz., the covering of the melt with a flux and the use of filter beds made of refractory material, represent the state of the art today. Both processes however suffer from disadvantages which make them difficult to use in practice.
In the process wherein a flux is used to cover the aluminum melt, in time enrichment of aluminum chloride occurs in this cover layer, forming with the alkali and alkaline earth halides complex salts which can be hydrolyzed by moisture in the air, which causes undesirable smokey fumes to form, and can also cause the viscosity of this layer to increase. This smoke consists mainly of hydrochloric acid and finely divided aluminum oxide and aluminum hydroxide. There is also the danger that the molten salt and its hydrolyzed products will diffuse into the furnace lining which can lead to corrosion problems there.
Because the distribution of the chlorine in the melt is poor, the efficiency of chlorine is usually too low for industrial application; this is also so because relatively expensive foundry equipment is needed to carry out the process.
A better distribution of gas in the melt is achieved by using filter beds made of ceramic materials. The major part of the aluminum chloride, which results from the reaction of the chlorine with the aluminum, precipitates out on the ceramic, which prevents to a large extent the development of smoke. As a result of the ceramic material being covered on all sides, and the precipitation of aluminum chloride on to it from the melt, no hydrolysis of the aluminum chloride from moisture in the air occurs and therefore there is no need to worry about corrosion of the furnace lining. The capacity of the ceramic material to store the sodium chloride formed is relatively small which makes it necessary to employ a filter bed of adequate dimensions which has correspondingly large heat losses.